Various types of radio frequency signal processing devices, such as Instantaneous Frequency Measurement (IFM) receivers or Digital Frequency Discriminators (DFDs), have been developed to detect specific RF signals in dense signal environments characteristic of electronic warfare. It is well known from the field of Electronic Counter Measures (ECM) to transmit a relatively weak information signal, (e.g. Radar) in the presence of a strong jamming signal having the same frequency as the information signal. Thus, ECM systems are designed to render extraction of the information signals extremely difficult, without knowing the jamming signal frequency.
Electronic Support Measurement (ESM) devices have been developed for extracting such relatively weak information signals in the presence of a strong jamming signal. One prior art approach consists of using a band-stop (or notch) filter for separating the information and jamming signals on the basis of small frequency differences therebetween. The band-stop filter is tuned to the frequency of the strong jamming signal, thereby attenuating it and allowing extraction of the information signal.
Obviously, this prior art approach requires a predetermined minimal amount of frequency separation between the jamming and information signals in order to attenuate the former without attenuating the latter. Thus, the previous approach suffers from two main disadvantages. Firstly, attenuation of the strong jamming signal is frequency dependent, such that in the event the frequency separation between the jamming and information signals is not sufficiently large with respect to the filter bandwidth, the information signal will suffer a certain amount of attenuation. Therefore, in the event the information and jamming signal frequencies are identical, the prior art approach cannot be used. Secondly, in the event the frequency of the strong jamming signal is unknown, the ESM device must first measure the jamming frequency and then tune the filter. Most ECM systems transmit jamming and information signals at a continuously variable frequency, according to a pseudorandom pattern, making frequency detection prior to filter tuning extremely difficult.